The present invention relates to a linear pulse motor and, more particularly, to a linear pulse motor suitable for a drive mechanism which is adapted to position a magnetic head at a destination track.
A linear pulse motor is used to move, for example, a magnetic head or an optical head linearly in a radial direction of a magnetic disk or an optical disk until the head reaches a destination track of the disk.
A linear pulse motor for such an application is disclosed in U.S. Pat. No. 4,578,622 patented Mar. 25, 1986 and issued to Nakagawa et al, especially in FIGS. 6 to 10 thereof. The linear pulse motor disclosed in the Patent includes a stationary body including four magnetic pole members and a movable body (secondary body) which is slidable linearly on and along the stationary body. A pair of first and second guide members are located at opposite sides of magnetic pole teeth of the stationary member to guide the movable body during the linear movement of the latter. While the first guide member is securely connected to the stationary body by rivets, the second guide member is allowed to move in a direction perpendicular to its lengthwise direction by the rivets and a leaf spring. The guide members are individually provided with ball guide portions each receiving a ball therein. The balls roll in the individual ball guide portions to cause the movable body to slide on the stationary body. A permanent magnet for applying a magnetic field to the teeth of the stationary body is located below the teeth. Further, a pair of magnetic core frames are juxtaposed to the permanent magnet to selectively cancel the magnetic field to thereby move the movable body, a coil being wound around each of the core frames.
In the conventional linear pulse motor described above, the movable body is linearly displaceable with the first guide member which is rigid on the stationary body serving as a reference. A prerequisite with such a structure is that the first guide member be accurately positioned on the stationary body by rivetting to insure accurate parallelism between the teeth of the stationary body and those of the movable body. However, since machining the magnetic pole members and first guide member with extreme accuracy is difficult, the teeth of the stationary body and those of the movable body fail to become parallel to each other. In such a condition, the static torque of the movable body changes depending upon the position where the movable body is stopped.
Further, in the conventional linear pulse motor, the dimension of a gap defined between the teeth of the stationary body and those of the movable body is determined by the four balls which roll on the four separate magnetic pole members. Hence, any error in the assembly of the magnetic pole members prevents a uniform gap from being defined between the teeth of the stationary body and those of the movable body in the lengthwise direction of the teeth.